Turrets that extend outwardly from a surface are utilized in a wide variety of applications in order to house one or more components of a system that needs visibility beyond the surface. For example, air vehicles, such as aircraft, may include one or more turrets that extend outwardly from the surface of the aircraft. These turrets may house components of various systems including, for example, components of a laser system, such as optical elements for receiving and steering a laser beam.
To allow orientation of the components within the turret over the complete range of orientations with respect to the aircraft to which the turret is mounted, a turret may have a shape generally described as “hemisphere on cylinder”. In this configuration, the cylinder rotates around an axis aligned with the cylinder's axis of symmetry. This axis is aligned generally perpendicular to the aircraft surface to which the turret is mounted. The hemisphere mounted to the cylinder rotates along an axis perpendicular to that around which the cylinder rotates. When used to project a laser beam, the final optical elements, including a window and telescope as required by the application, may be mounted internal to the hemispherical portion of the turret.
Turrets of this configuration create a region of relatively low flow velocity directly aft of the cylindrical portion of the turret. This reduced velocity may create separation of the ambient flow from the surface over which the ambient flow is otherwise propagating due to an adverse pressure gradient created by the relatively higher local pressure in this region of low flow velocity. In this regard, the ambient flow may propagate in a predefined direction relative to the surface, such as based upon the direction of movement of an aircraft and the prevailing wind currents. As such, the turret may create flow separation on the aft side of the turret relative to the predetermined direction of the ambient flow. By way of illustration, FIG. 1 depicts a turret 12 extending outwardly from the surface 14 of an aircraft. In an instance in which the predefined direction of the ambient flow extends in a direction generally from left to right as indicated by the arrows 16 in FIG. 1, the turret 12 may create flow separation in the region 18 aft of, that is, to the right of, the turret, relative to the predefined direction of the ambient flow. As shown in FIG. 1, the flow separation generally begins at about that portion of the turret 12 that is marked with a dashed line 20 and then continues for some distance downstream of the turret. This location at which flow separation begins and the size and shape of the region of flow separation is dependant on factors including the aircraft speed, turret size and design, and local atmospheric conditions.
Flow separation created by a turret 12 produces turbulence which may, in turn, effect the operation of the system housed within the turret. For example, a turret 12 that houses one or more components of a laser system may effect the operation of the laser system by creating the flow separation and, in turn, turbulence in the region 18 aft of the turret, since such turbulence may alter the precision of the laser system. Indeed, the turbulence may effect laser beam propagation, both as a result of the atmospheric turbulence in the propagation path of the laser beam and through the buffeting of the turret 12 by pressure fluctuation in the flow.
In order to avoid the issues created by turrets 12 that extend outwardly from a surface 14, the turret may be positioned on the nose of an aircraft or other air vehicle so as to eliminate flow separation and the attendant turbulence. However, only a limited number of aircraft or air vehicles can accommodate the mounting of a turret on the nose for reasons including structural limitations, interference with other critical systems and mass balance. Therefore turrets 12 that extend outwardly from the surface 14 of an air vehicle may oftentimes still be required.
As another alternative to a turret 12, a fairing may be utilized, but the fairing itself may create issues for the system housed therein. For example, in instances in which one or more components of a laser system are to be housed within a fairing, the fairing would generally be fabricated from a glass having relatively low absorption which may be difficult to obtain in the requisite size and/or may be expensive. Additionally, the fairing may alter the laser beam in a complex way due to its non-spherical shape, thereby further effecting the operation of the laser system.
Alternatively, the components of the system that are otherwise housed by a turret 12 may be disposed under or within the surface 14, such as within an aircraft, and may interact with the exterior environment through a conformal window. However, the use of such a conformal window may effect the field of regard of the system, such as a laser system, thereby altering system performance. Additionally, the use of a conformal window may increase costs and/or may require that additional space be dedicated to the system under or within the surface, such as within the aircraft.
As such, it may be desirable in a number of instances to utilize turrets 12 that extend outwardly from the surface 14 in order to house one or more components of a system, such as a laser system. In instances in which the ambient flow is at a relatively low Mach number (M), such as M=0.3 to 0.4, the turret may include fairing ramps that extend at least partially around the base of the turret proximate to the surface. These fairing ramps cause the velocity of the ambient flow to increase as it moves upwardly along the fairing ramp and around the turret, and can also induce vorticity into the flow. The fairing ramps may extend around both sides of the turret and may terminate with a tapered rear-facing step on the aft side of the turret. The termination of the fairing ramps may generate counter-rotating vortices that may produce suction aft of the turret due to their relatively high velocity with respect to the ambient flow around the turret. This suction may eliminate or at least reduce the adverse pressure gradient aft of the turret which, in turn, may prevent or at least reduce flow separation and the resulting turbulence aft of the turret.
While useful in instances in which the ambient flow has a relatively low Mach number, the fairing ramps may not efficiently reduce flow separation as the velocity of the ambient flow increases, such as to transonic speeds. In this regard, a transonic flow is generally a subsonic flow that may become sonic in one or more local regions. In this instance in which the ambient flow is transonic, the flow along the fairing ramps may create shock waves which, in turn, may create relatively large density gradients which may adversely affect the performance of the system housed by the turret.
As such, it may be desirable to provide an improved technique for reducing or eliminating flow separation aft of a turret that is operational at higher speeds, including, for example, transonic speeds in which the ambient flow has a Mach number of about 0.7 to 0.9. By reducing or eliminating the flow separation, the resulting turbulence can be similarly reduced or eliminated so as to avoid alteration of the performance of a system, such as a laser system, housed by the turret.